It is standard, for instance in a mill producing sheet steel, to transport a stack of plates by use of a magnetic grab. Such a grab is typically a powerful electromagnetic that is engaged with the topmost sheet of the stack and that is powerful enough to lift this top sheet and, through it, the remaining underlying sheets of the stack. Such magnetic grabs are also frequently used for other purposes, including of course lifting one-piece objects that are ferromagnetic, which term here is intended to cover any material capable of being attracted by a magnet. The advantage of such lifting is that it eliminates the need for moving parts and elements adapted to fit the workpiece or workpieces being moved, and can be made to grip when electrically energized and release when electrically de-energized.
More particularly when used to pick up ferromagnetic objects of large surface area such a system normally employ at least two more magnet assemblies in order to allow the magnetic holding forces necessary to act upon the ferromagnetic objects in a distributed manner across the surface of such ferromagnetic objects. To this end, the magnet assemblies usually each comprise two or more pole shoes by means of which a magnetic circuit can be closed through the ferromagnetic object being handled, thus resulting in a magnetic attractive force between the pole shoes and the at least one ferromagnetic object.
The magnetic grab may be configured, for example, in that it has a horizontal longitudinal beam carrying a plurality of transverse beams that are longitudinally equispaced. Respective magnet assemblies are mounted on the ends of the cross beams so that they form a rectangular array, although this is not always the case. Magnetic grabs deviating from this arrangement are also known, but normally have at least two magnet assemblies.
Magnet assemblies are also known in terms of their configuration in various embodiments. For example, magnet assemblies exist that are purely electromagnetic, therefore generating a holding force only when electrically energized. Thus, no magnetic holding force is generated by such magnet assemblies when not electrically energized si that such systems have the disadvantage of releasing their load in case of a power failure.
Hence, magnet assemblies are preferred that have a combination of permanent and electromagnets so that a magnetic field is constantly generated by a permanent magnet. There is therefore always some holding force.
In order to grip a ferromagnetic object without at the start applying any magnetic holding force to it, the magnetic holding force generated by the permanent magnet may be initially countered by electrically energizing the electromagnetic portion of such a magnet assembly to generate a magnetic force opposite that of the respective permanent magnet or magnets. The total magnetic holding force may then be particularly continuously increased by switching off or reducing the current applied, or optionally also by reversing the polarity of the applied current. Such a magnet assembly provides the advantage that even in case of power failure, and therefore in case of a loss of electromagnetic holding force, a holding force is still generated by the permanent-magnet element or elements of the magnet assembly.
It is known that the magnetic holding force substantially depends on the magnetic flux generated by a magnet assembly connected to the at least one ferromagnetic object. To this end, however, the magnetic flux and the magnetic holding force generated by means of the same are not identical in all magnet assemblies with the use of multiple magnet assemblies, even those having an identical construction and identical current feed of such magnet assemblies.
This is due to the fact that different magnetic resistances exist in the magnetic circuit generated between the magnet assembly and the at least one ferromagnetic object. The magnetic resistances may be influenced, for example, by air gaps, material qualities of the ferromagnetic objects or magnet assemblies to be lifted, temperatures of the ferromagnetic objects and the magnet assemblies, and particularly also on the surface structures of the at least one ferromagnetic object that may vary, for example, due to scaling, rust, coatings, unevenness, etc.
Particularly when gripping, lifting, and transporting a plurality of ferromagnetic objects, such as when gripping, lifting, and transporting a stack of flat sheets, these effects are amplified, as the above-described variables for each of the individual flat sheets contributing to the magnetic resistances are present individually.
Therefore, significant deviations between the individual holding forces of the magnet assemblies may occur during the gripping, lifting, and transporting of ferromagnetic objects, particularly if they are not compact, and particularly when gripping a stack of multiple flat sheets, most particularly if the magnet assemblies are not activated in an identical manner.
Another effect in flat sheets is that they do not stay planar after being gripped and lifted. More particularly, after gripping, that is when lifted and being transported, they usually bend, and the amount of bend increases when only a few of the magnetic grabs are used. Such bend is responsible for a significant reduction of the holding force, particularly due to an enlargement of the air gap between individual flat sheets.
This results in the risk in commonly used methods or systems for gripping, lifting, and transporting of ferromagnetic objects that the different magnetic holding forces applied by the magnet assemblies deviate from each other. As a result, some of the magnet assemblies fall short of a necessary minimum holding force may be present, and a ferromagnetic object being carried may be dropped. Particularly when gripping, lifting, and transporting a stack of flat sheets the lowermost sheet may bend and drop off because of the locally different holding forces of the magnet assemblies.